The present invention relates to a tilt cylinder controller in an industrial vehicle such as a forklift. More particularly, the present invention pertains to a controller that controls tilt cylinders, which tilt a mast that supports a load carrier such as a fork.
A typical industrial vehicle such as a forklift includes a mast pivotally supported on the front of the vehicle. The forklift also has a fork supported by the mast to be lifted and lowered. A lift lever is provided in the forklift's cab. An operator manipulates the lift lever to actuate lift cylinders thereby lifting and lowering the fork. A tilt lever is also provided in the cab. The operator manipulates the tilt lever to actuate tilt cylinders thereby tilting the mast forward or rearward.
When a load is on the fork, the center of gravity of the forklift is moved forward. Increasing the height of the fork increases the moment acting on the mast. Tilting the mast forward with a load on the fork further moves the center of gravity forward and thus destabilizes the forklift. Also, if a heavy load is on the fork and the mast is tilted rearward by a great angle, the forklift's center of gravity is moved rearward. This may cause a front wheel of the forklift to lose contact with the road surface and to spin. Therefore, the maximum forward tilt angle of the mast is typically set at six degrees, and the maximum rearward tilt angle is set at twelve degrees.
When removing a load from the fork to an elevated place, the mast is tilted forward with the fork raised. If the mast is tilted too quickly by an excessive angle, the load on the fork may shift and the rear wheels may lose contact with the road surface. Thus, the operator must carefully control the mast so that the mast is slowly tilted forward by a sufficiently small angle. This requires experience.
When loading and unloading the fork, the fork needs to be parallel with a pallet for carrying a load. In other words, the fork must be leveled. However, tilting of the mast supporting the fork is typically controlled by a manually controlled valve. That is, the operator manipulates the manual valve using the tilt lever thereby controlling flow of hydraulic oil from and into the tilt cylinders. Manipulating the tilt lever for accurately leveling the fork therefore requires experience. Further, the operator usually manipulates the tilt lever and the lift lever while driving the forklift. This makes operation of a forklift more difficult.
To facilitate operation, some forklifts are equipped with an electromagnetic valve, instead of a manual valve, for regulating oil flow from and into the tilt cylinders. The electromagnetic valve allows an operator with little experience to accurately control the tilting of mast. The electromagnetic valve also allows the operator to easily level the fork.
There are also forklifts that have an automatic stopping device for preventing the forklift from operating when the operator is not sitting on the seat in the cab. The stopping device detects whether the operator is sitting on the seat by using a sensor and prohibits the forklift's operation if the operator is not sitting on the seat.
However, if oil flow from and into tilt cylinders is controlled solely by an electromagnetic valve, the electromagnetic valve needs to be large and complex. This increases the manufacturing cost. Electromagnetic valves are spool type valves. A spool type valve includes a housing and a spool slidably housed in the housing. The spool has a circumferential surface slidably contacting the housing. A narrow clearance exists between the circumferential surface of the spool and the housing such that the spool moves smoothly in the housing. When a relatively great force acts on the valve, the clearance causes oil leakage. Compared to manual valves, the clearance in electromagnetic valves is large for allowing the spool to move smoothly. The larger clearance increases the amount of oil leakage.
If a forklift has an automatic stopping device and an electromagnetic valve for controlling the tilt cylinders, the tilt cylinders are immediately stopped when the operator leaves the seat. However, when tilting the mast with a bulky load on the fork, the operator may have to half-rise from the seat to look to the front. If the operator half-rises, the automatic stopping device immediately stops the tilt cylinders. The operator then has to sit on the seat again for resuming the operation. This results in inefficient operation of the forklift.